Amlodipine, a generic name for the compound of formula (2), 3-ethyl-5-methyl-2-(2-aminoethoxy-methyl)-4-(2-chlorophenyl)-6-methyl-1,4-dihydro-3,5,-pyridine dicarboxylate, is a long-term calcium-channel blocker useful for treating cadiovacular diseases such as stenocardia, hypertension and congestive cardioplegic. 
European Patent Publication No. 89167 discloses various different types of pharmaceutically acceptable salts of amlodipine. A pharmaceutically acceptable salt is made by adding a pharmaceutically acceptable acid to form a non-toxic salt of amlodipine acid type, and examples thereof include hydrochloride, hydrobromide, sulfate, phosphate or acidic phosphate, acetate, maleate, fumarate, lactate, tartarate, citrate and gluconate. Among these salts, maleate is most preferable.
Amlodipine in the form of a free base is useful for pharmaceutical use. However, because amorphous amlodipine shows a low stability, it is preferable to administer in the form of a salt of a pharmaceutically acceptable acid.
Korean Patent Publication No. 95-6710 suggests that a pharmaceutically acceptable salt must meet four physicochemical requirements: high solubility, good stability, non-hygroscopicity and processibility for tablet formulation.
Most amlodipine salts are amorphous and it is difficult to prepare purely them. Further, an acid-added salt of amlodipine that meets all the above requirements is yet to be developed. For example, it has been found that even amlodipine maleate, which is proposed as the most preferable pharmaceutical form of amlodipine, has a relatively high solubility in water, but degrades in a solution within several weeks.
Korean Patent Publication No. 95-7228 discloses that amlodipine benzenesulfonate (hereinafter, “amlodipine besylate”) shows a high solubility and good stability, and has suitable properties for preparing a pharmaceutical formulation. However, amlodipine besylate is derived from toxic benzene sulfonic acid, and therefore, there exists a safety issue.
The present inventors have endeavored to develop a novel crystalline amlodipine that satisfies all the require properties.